Olefins, also called alkenes, are unsaturated hydrocarbons whose molecules contain one or more pairs of carbon atoms linked together by a double bond. When subjected to a polymerization process, olefins are converted to polyolefins such as polypropylene. One commonly used polymerization process involves contacting the olefin monomer with a Ziegler-Natta catalyst system that includes a conventional Ziegler-Natta pre-catalyst (also referred to herein as a pre-catalyst component), a co-catalyst, and one or more electron donors. Examples of such catalyst systems are provided in U.S. Pat. Nos. 4,107,413; 4,294,721; 4,439,540; 4,114,319; 4,220,554; 4,460,701; 4,562,173; and 5,066,738, which are incorporated by reference herein.
The pre-catalyst component of a conventional Ziegler-Natta catalyst system is comprised of a transition metal compound supported on an inert solid such as a magnesium compound. The transition metal compound is generally represented by the formula:MRxwhere M is a transition metal, R is a halogen, an alkoxy, or a hydrocarboxyl group, and x is the valence of the transition metal. Typically, M is a group IV–VIB metal such as titanium, chromium, or vanadium, and R is chlorine, bromine, carbonate, ester, or an alkoxy group. Common transition metal compounds are TiCl4, TiBr4, Ti(OC2H5)3Cl, Ti(OC3H7)2Cl2, Ti(OC6H13)2Cl2, Ti(OC2H5)2Br2, and Ti(OC12H25)Cl3.
An internal electron donor is typically added to the Ziegler-Natta pre-catalyst during its preparation and can be combined with the support or otherwise complexed with the transition metal compound. Examples of internal electron donors include amines, amides, esters, ketones, nitriles, ethers, and phosphines. The internal electron donor is used to reduce the atactic form of the resulting polymer, thus decreasing the amount of xylene solubles fraction of the produced resin. A polymer is “atactic” when its pendant groups are arranged in a random fashion on both sides of the chain of the polymer. In contrast, a polymer is “isotactic” when all of its pendant groups are arranged on the same side of the chain and “syndiotactic” when its pendant groups alternate on opposite sides of the chain. Isotactic and syndiotactic polyolefins have better mechanical properties than atactic polyolefins. For example, isotactic and syndiotactic polyolefins, unlike atactic polyolefins, can be formed into crystals and fibers because the regular arrangement of their atoms allows them to be easily packed together.
During the polymerization process, an external electron donor may be added as another component of the catalyst system to further control the amount of atactic polymer produced. Examples of commonly used external electron donors include organosilicon compounds, such as diphenyldimethoxysilane (DPMS), cyclohexylmethyldimethoxysilane (CMDS), and dicyclopentyldimethoxysilane (CPDS). A co-catalyst, such as an organoaluminum compound, also may be used in conjunction with the Ziegler-Natta pre-catalyst to activate the catalyst system.
In the polymerization process, hydrogen is fed to the catalyst system to terminate the chain formation of the polymer, thereby controlling the molecular weight and the melt flow rate of the polymer. The hydrogen response of the Ziegler-Natta catalyst system affects the molecular weight of the polymer produced. In particular, an increase in hydrogen response produces a lower molecular weight polymer (i.e., shorter chain length), and a decrease in hydrogen response produces a higher molecular weight polymer (i.e., longer chain length). As molecular weight decreases, the melt flow rate (MFR) of the polymer increases. Polyolefins having relatively high MFR values offer numerous processing advantages. For example, lower temperatures and lower die pressures are required for the extrusion of such polyolefins. Further, the use of such polyolefins reduces the wear on the extrusion equipment. A Ziegler-Natta catalyst system having a relatively high hydrogen response is thus needed to better control the molecular weights of the polymers produced using the catalyst.
The properties of the polymerization catalyst system affect the properties of the polymer formed. For example, polymer morphology typically depends upon catalyst morphology. Good polymer morphology includes uniformity of particle size and shape and minimization of the number of very small particles (i.e., fines) in the polymer to avoid plugging process transfer and recycle lines. Very large particles also must be minimized to avoid formation of lumps and strings in the polymerization reactor. Unfortunately, modification of conventional supported catalysts to optimize morphology typically sacrifices the original activity and stereospecificity of the catalysts.
The present invention provides a process for forming a Ziegler-Natta catalyst system that can be used to produce polyolefins with desired properties. The pre-catalyst formed in accordance with the present invention is highly active and has a satisfactory morphology that can be controlled by varying the amount of phosphorous compound used to form the catalyst system. Furthermore, the catalyst has a relatively high hydrogen response, allowing for better control of the molecular weight of the polymer chain. The polypropylene also contains a low level of xylene solubles, which indicates that the atactic form of the polymer is also low.